linux-kernel - Re: [PATCH v2 1/1] cpuidle: menu: Use min() to prevent deep C-states when tick is stopped

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Message-ID: <20260122111959.14e8fb3e@pumpkin>
Date: Thu, 22 Jan 2026 11:19:59 +0000
From: David Laight <david.laight.linux@...il.com>
To: "Ionut Nechita (Sunlight Linux)" <sunlightlinux@...il.com>
Cc: rafael@...nel.org, daniel.lezcano@...aro.org, christian.loehle@....com,
 linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 yumpusamongus@...il.com, Ionut Nechita <ionut_n2001@...oo.com>,
 stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] cpuidle: menu: Use min() to prevent deep
 C-states when tick is stopped

On Thu, 22 Jan 2026 10:09:39 +0200
"Ionut Nechita (Sunlight Linux)" <sunlightlinux@...il.com> wrote:

> From: Ionut Nechita <ionut_n2001@...oo.com>
> 
> When the tick is already stopped and the predicted idle duration is short
> (< TICK_NSEC), the original code uses next_timer_ns directly. This can
> lead to selecting excessively deep C-states when the actual idle duration
> is much shorter than the next timer event.
> 
> On modern Intel server platforms (Sapphire Rapids and newer), deep package
> C-states can have exit latencies of 150-190us due to:
> - Tile-based architecture with per-tile power gating
> - DDR5 and CXL power management overhead
> - Complex mesh interconnect resynchronization
> 
> When a network packet arrives after 500us but the governor selected a deep
> C-state (PC6) based on a 10ms timer, the high exit latency (150us+)
> dominates the response time.
....

We had to disable the deep sleep states on much older Intel -7 cpus.
The problem was that we needed to wake up multiple cpu and they tended
to get woken in turn - so it was far too long before they were all running.
I suspect that pretty much anything that cares about latency has always
needed to disable them.

	David